Between the positive electrode and the negative electrode of a chemical battery such as a lithium secondary battery, a separator is disposed, which serves to provide an electrical insulation between the electrodes, and to retain the electrolyte. For lithium secondary batteries, separators comprising a polyolefin resin such as polyethylene or polypropylene are mainly used at present. In general, the positive electrode and the negative electrode are each formed by a current collector and an active material layer carried on the current collector.
However, such separators comprising a polyolefin resin tend to contract when exposed to a high temperature environment, so that there is the possibility that the positive electrode and the negative electrode come into physical contact with each other, causing internal short circuit. Particularly, when internal short circuit occurs between the positive and negative electrode current collectors, or between the positive electrode current collector and the negative electrode active material, a larger amount of short circuit current flows owing to a low resistance at the short circuit portion, resulting in a high possibility of causing temperature increase in the battery.
Furthermore, with the recent demand for high capacity lithium secondary batteries, there is a tendency to reduce the thickness of separators. Thus, the problem of internal short circuit is becoming increasingly important.
Therefore, in order to address the above-described problems and to improve the safety of the battery, there has been proposed a technique for preventing internal short circuit between the positive electrode current collector and the negative electrode current collector by attaching an insulating tape to an exposed portion of the positive or negative electrode current collector (see Japanese Laid-Open Patent Publication No. 2004-247064). There has also been proposed a technique for forming an ion-permeable, heat-resistant layer comprising ceramic particles and a binder on at least one of the positive electrode plate and the negative electrode plate, in order to prevent short circuit between the positive electrode and the negative electrode (see Japanese Laid-Open Patent Publication No. Hei 10-106530).
However, in order to prevent short circuit between the exposed portion of the positive electrode current collector and the negative electrode only by attaching the insulating tape, the exposed portion of the current collector must be completely covered with the insulating tape in the technique disclosed in Japanese Laid-Open Patent Publication No. 2004-247064. However, an extremely high precision is required for attaching the insulating tape so as to completely cover only the exposed portion of the positive electrode current collector. This leads to a reduced productivity. On the other hand, when the insulating tape is attached onto an area extending from the exposed portion of the positive electrode current collector to the positive electrode active material layer, the part of the positive electrode active material layer onto which the insulating tape is attached cannot contribute to the battery reaction. This will result in a decreased battery capacity.
When the heat-resistant layer is formed, for example, on the negative electrode with the technique disclosed in Japanese Laid-Open Patent Publication No. Hei 10-106530, it is necessary to reliably form the heat-resistant layer also on the boundary portion between the negative electrode active material layer and the exposed portion of the negative electrode current collector, in order to reliably prevent short circuit between the exposed portion of the positive electrode current collector and the negative electrode. However, since there is a height difference between the negative electrode active material layer and the exposed portion of the negative electrode current collector, it is necessary to decrease the speed of applying a paste for forming a heat-resistant layer, in order to reliably form the heat-resistant layer on their boundary portion. This leads to a reduced productivity.
In view of the foregoing problems, it is an object of the present invention to provide a lithium secondary battery that realizes both improved safety such as high short circuit resistance and heat resistance, and high battery capacity, while providing excellent productivity.